Multi-function composite optical film

ABSTRACT

A composite optical film has the functions of brightness enhancement, self-diffusion and defect blocking. The film comprises: a structured substrate including a structured light incidence surface and a light emitting surface opposite the structured light incidence surface, wherein the structured light incidence surface includes longitudinal prism structures or longitudinal lens structures transversely arranged in rows; a support base plate adjacent to the light emitting surface of the structured substrate, the support base plate having a light incidence surface on one side and a light emitting surface on another side opposite the light incidence surface, wherein at least one of the light incidence surface and the light emitting surface has a plurality of buried particles, and at least one portions of the buried particles are protruded on the light emitting surface of the support base plate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No. 14/269,481 filed on May 5, 2014, which claims priority from Taiwan Patent Application No. 102118879 filed on May 29, 2013. All of these applications are incorporated by referenced herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a composite optical film including a structuralized substrate, and in particular, to an optical film capable of enhancing brightness, automatically diffusing light and blocking defects, and further in particular, to a composite optical film applicable to a flat panel display.

2. Description of Related Art

Flat panel displays are widely used for television displays, computer displays, and the displays of hand-held electronic apparatuses (e.g. mobile phones, mobile internet devices (MID) and so on). Liquid crystal displays (LCDs) pertain to a kind of the flat panel displays. LCDs display images by using a liquid crystal module having a pixel array.

FIG. 1 illustrates a conventional LCD. A backlight LCD 10 comprises a liquid crystal display module 12 and a backlight module 14 forming a plane light source. The backlight LCD 10 further comprises several optical films such as an upper light diffusion sheet 22, a lower light diffusion sheet 24 and brightness enactment sheets (26, 28) interposing between the liquid crystal display module 12 and the backlight module 14. The liquid crystal display 12 comprises liquid crystals sandwiched between two transparent substrates and a control circuit for defining a two-dimensional pixel array. The backlight module 14 has a plane light distribution and serves as a backlight directing light on a plane. As shown in FIG. 1, an edge-type backlight has a linear light source 16 disposed along an edge of a light guide plate 18. A reflecting sheet 20 is used to direct light from the linear light source 16 into the light guide plate 18 so that the light passes the uppermost surface of the liquid crystal display module 12. The optical films of the upper diffusion sheet 22, the lower diffusion sheet 24 and the brightness enactment sheets (26, 28) can redistribute or concentrate the through light. Thus, the light leaving from the optical films can be directed so that it goes towards a direction quite close to the normal line of the surface of the liquid crystal display module 12. The light is evenly distributed on the plane surface of the liquid crystal display module 12 and has a relatively high intensity along the normal line direction.

Power saving, less thickness and lightness is increasingly in demand for the liquid crystal display under constant quality. Therefore, the power dissipation, weight and thickness of the backlight module would be expectably reduced, and the thicknesses of the optical films therein would be also accordingly reduced. Most conventional liquid crystal displays have used two brightness enhancement sheets (as shown in the LCD of FIG. 1). Two brightness enhancement sheets are arranged by rotating them around the normal lines of the film planes so that the longitudinal directions of grooves on the two films are perpendicular to each other. Therefore, the light through the two films would be concentrated along the direction perpendicular to their light output surfaces. When optical turbulence occur in one of the flat surfaces or structuralized surfaces of the two brightness enhancement sheets, a displayed image consequently has undesired visual effects such as interference fringes (i.e. patterns having interlaced bright and dark stripes). The undesired visual effects resulting from the interference fringes, physical defects, spots and unequal distribution can be masked by an upper diffusion sheet 22 (e.g. the upper light diffusion sheet 22 above the brightness enhancement sheet 26).

Up to now, in order to reduce the entire thickness of all optical films in a liquid crystal display, a lot of technologies have focused on reducing the number of optical films used therein. For example, four optical films (FIG. 1 illustrating the optical films 22, 24, 26 and 28) are reduced to three optical films. For a common case, the upper light diffusion sheet 22 and the brightness enhancement sheet 26 are integrated into a single composite optical film, and the single composite optical film is further combined with the lower light diffusion sheet 24 and the brightness enhancement sheet 28 into a three-sheet structure. A liquid crystal display using such a three-sheet structure is widely employed in hand-held electronic apparatuses and laptop computers.

Further, in order to reduce the dimension and the cost of such products, the display manufacturers still need a liquid crystal display with a lighter and cost-effective optical structure superior to one with a three-sheet structure. Such an optical structure is capable of enhancing brightness, automatically diffusing light and masking defects.

SUMMARY OF THE INVENTION

The present application provides a composite optical film has the functions of brightness enhancement, self-diffusion and defect masking. The film comprises: a structured substrate including a structured light incidence surface and a light emitting surface opposite to the structured light incidence surface, wherein the structured light incidence surface includes longitudinal prism structures or longitudinal lens structures transversely arranged in rows; a support base plate adjacent to the light emitting surface of the structured substrate, the support base plate having a light incidence surface on one side and a light emitting surface on another side opposite to the light incidence surface, wherein at least one of the light incidence surface and the light emitting surface has a plurality of buried particles, and at least one portion of the buried particles are protruded on the light emitting surface of the support base plate. In one embodiment of the present invention, the longitudinal prism structures or the longitudinal lens structures are transversely arranged in row, wherein the crests of the prism structures or the lens structures in adjacent rows are substantially parallel to each other and distances between the adjacent crests are constant.

In one embodiment of the present invention, the heights of the crests of the prism structures or the lens structures are constant.

In one embodiment of the present invention, the heights of the crests of the prism structures or the lens structures vary along a longitudinal direction.

In a further embodiment of the present invention, the haze of the film is adjustable by varying the number of particles buried in at least one of the light incidence surface and the light emitting surface of the support base plate and the number of particles protruded on the light emitting surface of the support base plate.

In another embodiment of the present invention, the surface roughness, scratch-resistance and transmittance of the film are adjustable by varying the number of particles protruded on the light emitting surface of the support base plate.

In another embodiment of the present invention, the prism structures or the lens structures are formed using an UV resin material so that the structured substrate is capable of scratch-resistance and self-healing.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to sufficiently understand the essence, advantages and the preferred embodiments of the present invention, the following detailed description will be more clearly understood by referring to the accompanying drawings.

FIG. 1 schematically illustrates the structure of a prior art LCD;

FIG. 2 is a schematic perspective view of a composite optical film having a structured light incidence surface, in accordance with one embodiment of the present invention;

FIG. 3a is a schematic perspective view of a composite optical film having longitudinal prism structures transversely arranged in rows, in accordance with one embodiment of the present invention;

FIG. 3b is a schematic perspective view of a composite optical film having longitudinal lens structures transversely arranged in rows, in accordance with one embodiment of the present invention;

FIG. 3c is a schematic perspective view of a composite optical film having longitudinal prism structures with varied height transversely arranged in rows, in accordance with one embodiment of the present invention;

FIG. 3d is a schematic cross-sectional view of the composite optical film taken along a B-B line in FIG. 3 c;

FIG. 3e is a schematic perspective view of the flipped composite optical film with an angle of 180° in FIG. 3 c;

FIG. 4a schematically illustrates a flat panel display, in accordance with one embodiment of the present invention;

FIG. 4b schematically illustrates a flat panel display, in accordance with another embodiment of the present invention;

FIG. 5a is a schematic perspective view of a light guide plate having longitudinal prism structures with varied height transversely arranged in rows, in accordance with one embodiment of the present invention;

FIG. 5b is a schematic perspective view of a light guide plate having longitudinal lens structures transversely arranged in rows, in accordance with one embodiment of the present invention;

FIG. 5c is a schematic perspective view of a light guide plate having longitudinal prism structures with varied height transversely arranged in rows, in accordance with one embodiment of the present invention;

FIG. 5d is a schematic cross-sectional view of the light guide plate taken along a C-C line in FIG. 5c ; and

FIG. 6 is an SEM photograph of a light emitting surface with a plurality of buried particles.

DETAILED DESCRIPTION OF THE INVENTION

The following description shows the preferred embodiments of the present invention. The present invention is described below by referring to the embodiments and the figures. Thus, the present invention is not intended to be limited to the embodiments shown but is to be accorded the principles disclosed herein. Furthermore, that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.

The drawings show a whole configuration of each embodiment in a simple and clear manner. Well known features may not have been described in detail to avoid unnecessarily obscuring the invention. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.

The invention relates to a composite optical film capable of enhancing brightness, automatically diffusing light and masking defects. The composite optical film of the present invention is in the form of a film, sheet, plate, and the like, which may be flexible or rigid. The composite optical film of the present invention has a structured light incidence surface including longitudinal prism structures or longitudinal lens structures transversely arranged in rows. The inventive optical film may be adopted in display devices having display panels that may be flat or curved, and rigid or flexible.

The present invention will be further described below in connection with the illustrated embodiments.

FIG. 2 schematically illustrates an example of a flat panel display. A backlight LCD 100, in accordance with one embodiment of the present invention, comprises a liquid crystal (LC) display module 112, a planar light source in the form of a backlight module 114 and a composite optical film 124 interposed between the LC module 112 and the backlight module 114. The LC module 112 comprises liquid crystals sandwiched between two transparent substrates and a control circuit defining a two-dimensional array of pixels. The backlight module 114 provides planar light distribution. The backlight module 114 is either of the backlit type (i.e. the light source extends over a plane) or of the edge-lit type as shown in FIG. 2 (i.e. a linear light source 116 is provided at an edge of a light guide plate 118). A reflector 120 is provided to direct light from the linear light source 116 through the edge of the light guide 118 into the light guide 118. The light guide plate 118 is structured (e.g., with a tapered or flat plate having light reflective and/or scattering surfaces defined on the bottom surface facing away from the LC module 112) to distribute and direct light through the top planar surface facing towards LC module 112.

FIG. 3a is a schematic and perspective cross-sectional view of the composite optical film 124 taken along an A-A line in FIG. 2. FIGS. 3b and 3c are schematic and perspective cross-sectional views of composite optical films 124 b and 124 c. They respectively represent three embodiments of the composite optical films. However, the present invention is not limited to the ratio of dimensions as illustrated in the embodiments and the arrangement and combination ways of the microstructures. As shown in FIG. 3a , a composite optical film 124 comprises a structured substrate 125 and a support base plate 126. The structured substrate 125 includes a structured light incidence surface 125 a and a light emitting surface 125 b opposite to the structured light incidence surface 125 a. The structured light incidence surface 125 a includes longitudinal prism structures (as shown in FIG. 3a ) or longitudinal lens structures (as shown in FIG. 3b ; the structured light incidence surface 125 a′ of the structured substrate 125′ looks smoothly wavelike) transversely arranged in rows. The crests (or peaks) of the prism structures in adjacent rows are substantially parallel to each other and the pitches P1 between the adjacent crests are constant. The structured substrate 125 may be formed using an optically transparent material such as an ultraviolet (UV) polymerizable resin or a UV elastic resin material. The support base plate 126 is adjacent to the light emitting surface 125 b of the structured substrate 125 and has a light incidence surface 126 a on one side and a light emitting surface 126 b on another side opposite to the light incidence surface 126 a. The support base plate 126 may be made of a transparent material, such as polyethylene-terephthalate (PET), acryl, polycarbonate (PC), polyethylene naphthalate (PEN), other polymers known by a person ordinarily skilled in the art, and a compound of foregoing polymers or a copolymer of them. At least one of the light incidence surface 126 a, the light emitting surface 126 b and the middle of the support base plate 126 includes a plurality of buried particles 127 which may be organic particles or inorganic particles. The buried particles 127 may be formed using polymethyl methacrylate (PMMA), silica, silicone, polystyrene (PS), melamine, BaSO₄, CaCO₃, TiO₂, ZrO₂, AlO₃, silica gel, and a combination mixture of the foregoing materials. The shape of the particles may be spherical, ellipsoidal, grape-string-like, hollow or polyhedral. The buried particles 127 can scatter light and redistribute some passed light so that the light emitted from the composite optical film 124 can be directed forward along the normal line of the light emitting surface 126 b. The bead loading of the buried particles 127 and the number of the buried particles 127 protruded on the light emitting surface 126 b affect the roughness, scratch-resistance and transmittance of the film. The requirement for the haze (3-85%) of the composite optical film 124 can be met by varying the bead loading (about 2-50%) of the buried particles 127 protruded on the light emitting surface 126 b. The height of the protruded portion of the particle is about one fifth to two fifths of its diameter. FIGS. 3a and 3b illustrate the light emitting surface 126 b including a plurality of buried particles 127 in accordance with two embodiments of the application. FIG. 6 is an SEM photograph of a light emitting surface 126 b with a plurality of buried particles 127.

In these embodiments, the heights of the crests of the longitudinal prism structures of the composite optical film 124 or the heights of the crests of the longitudinal lens structures of the composite optical film 124 b are constant (as shown in FIGS. 3a and 3b ). The heights of the crests of the lens structures differ or vary by h1 along their longitudinal directions, as shown in FIG. 3c . FIG. 3d is a schematic cross-sectional view of the composite optical film taken along a B-B line in FIG. 3c . The higher crest of the prism structures along their longitudinal directions is H1 in height, and the lower crest of them is H2 in height. The difference between H1 and H2 is h1 (i.e. H1−H2=h1). FIG. 3e is a schematic perspective view of the flipped composite optical film with an angle of 180° in FIG. 3c . This figure shows that the crests of the lens structures differ or vary in height along their longitudinal directions. In another embodiment, FIGS. 4a and 4b illustrate flat panel displays 100 a and 100 b. As shown in FIG. 4a , the light guide plate 118′ comprises a structured light emitting surface 118 a which includes a plurality of longitudinal prism structures transversely arranged in rows. Further, as shown in FIG. 4b , the light guide plate 118″ comprises a structured light emitting surface 118 a′ which includes a plurality of longitudinal lens structures transversely arranged in rows. The structured light emitting surfaces 118 a and 118 a′ are respectively used to scatter light and enhance brightness. The light is directed outwards from the light guide plates 118′ and 118″. The structured light incidence surfaces 125 a and 125 a′ of the composite optical films 124 and 124 b respectively receive the light emitted from the structured light emitting surfaces 118 a and 118 a′.

In the foregoing embodiments, as shown in FIG. 5a , the light guide plate 118′ comprises a structured light emitting surface 118 a which includes longitudinal prism structures transversely arranged in rows. Furthermore, as shown in FIG. 5b , the light guide plate 118″ comprises a structured light emitting surface 118 a′ which includes a plurality of longitudinal lens structures transversely arranged in rows. The crests of the prism structures or the lens structures in adjacent rows are substantially parallel to each other and the pitches p2 between the adjacent crests are constant. As shown in FIGS. 5a and 5b , the heights of the crests of the longitudinal prism structures or the heights of the crests of the longitudinal lens structures are constant. The heights of the crests of the lens structures vary by h2 along their longitudinal directions, as shown in FIG. 5c . FIG. 5d is a schematic cross-sectional view of the light guide plate taken along a C-C line in FIG. 5c . The higher crest of the prism structures along their longitudinal directions is H3 in height, and the lower crest of them is H4 in height. The difference between H3 and H4 is h2 (i.e. H3−H4=h2).

In another embodiment, the longitudinal prism structures or lens structures transversely arranged in rows of the composite optical film 124 and the longitudinal lens structures transversely arranged in rows of the light guide plate 118 are arranged by aligning them at a relative angle of 90° so that the entire brightness is optimal. Furthermore, in order to eliminate the interference fringes or moire patterns occurring in the pixel array of the optical films, the plates and the liquid crystal module, in another embodiment, the relative angle between two aligned optical pieces can be below 90° by adjusting them, but the entire brightness is accordingly reduced. The aligned angle between the composite optical film having longitudinal lens structures and the light guide plate having longitudinal lens structures affects the entire brightness in a manner listed in Table 1-1. In another embodiment, the brightness of the composite optical film is changeable by adjusting its haze. The brightness of the composite optical film is reduced as its haze is increased. However, referring to Table 1-2, as the haze is increased, the composite optical film can have better effects on defect masking and optical uniformity for the module, but the assembly of the composite optical film and the plane light guide plate cannot resist wet-out. In another embodiment, Table 1-3 shows the relation between the haze and the entire brightness of the composite optical film whose heights of the crests of the prism structures vary along a transversal direction. The combination of such a composite optical film and a flat light guide plate can resist wet-out.

TABLE 1-1 the relation of the aligned angle and the entire brightness between the composite optical film and the light guide plate aligned angle prism prism angle haze gain of (degree) pitch (um) (degree) (%) brightness 90 24 68 5 9.01 86 24 68 5 8.97 82 24 68 5 8.53 78 24 68 5 8.26 74 24 68 5 7.86 70 24 68 5 6.75

TABLE 1-2 the relation of the haze and the entire brightness between the composite optical film prism prism aligned haze pitch angle thickness angle gain of defect optical (%) (um) (degree) (um) adhesion (degree) brightness hardness masking uniformity wet-out 5 24 68 156 ± 5 5B 90 9.01 2H 3 3 No 12 24 68 158 ± 5 5B 90 8.72 2H 3 3 No 30 24 68 155 ± 5 5B 90 7.33 3H 5 5 No 45 24 68 159 ± 5 5B 90 6.02 3H 5 5 No

TABLE 1-3 the relation of the haze and the entire brightness between the composite optical film resisting wet-out prism haze pitch prism thickness aligned gain of defect optical (%) (um) angle (um) adhesion angle brightness hardness masking uniformity wet-out 5 24 68 154 ± 5 5B 90 8.90 2H 3 3 Yes 12 24 68 156 ± 5 5B 90 8.25 2H 3 3 Yes 30 24 68 157 ± 5 5B 90 7.13 3H 5 5 Yes

In other embodiments, the haze of the composite optical film 124 can be controlled to 3-85% according to the thickness of the support base plate 126, the prism structure or the angle, height, and pitch of the crest of the lens structure, the amount of buried particles, and the amount of protruded particles.

In other embodiments, the thickness of the support base plate 126 may be 25 μm, 38 μm, 50 μm, 75 μm, 100 μm, 125 μm, 188 μm, 250 μm, or 300 μm. The thickness of the support base plate formed using an UV (ultraviolet) curable adhesive may be 10-100 μm, preferably 12-40 μm.

Table 2 shows the characteristics of support base plates with various thicknesses and prism structures with various thicknesses such as brightness gain, high scratch-resistance and so on. The adhesion listed in the table is measured under the test standard of ASTM D3359-95a which is used to determine the adhesion between the prism structure layer and the support base plate. The prism structures are divided into 100 small areas using a cross cutter along longitudinal and transverse directions, and then a 3M tape with model No. #600 is attached to them and pulled up. A magnifying glass is used to inspect the adhesion condition of the laterals of the 100 areas. The determination standard is shown in Table 3.

TABLE 2 the characteristics of support base plates with various thicknesses and prism structures with various thicknesses such as brightness gain, high scratch-resistance and so on thickness Thickness of support of base prism composite aligned plate haze prism angle film angle gain of defect optical (um) (%) pitch (degree) (um) adhesion (degree) brightness hardness masking uniformity 125 30 50 68 167 ± 5  5B 90 7.74 3H 5 5 188 30 50 68 229 ± 8  5B 90 7.56 3H 5 5 250 30 50 68 292 ± 10 5B 90 7.24 3H 5 5

In other embodiments, the structured substrate 125 has longitudinal prism structures arranged in rows. The pitch between the prisms may be 20-100 μm, preferably 20-60 μm, more preferably 20-40 μm, most preferably 20-30 μm. The angle of the prism may be acute and range from 40-120 degrees, preferably 50-100 degrees, more preferably 60-90 degrees, most preferably 60-75 degrees. The angle of the prism may be rounded off with a radius of 0.1-5 μm, preferably 0.3-1 μm.

In other embodiments, the structured substrate 125 has longitudinal lens structures arranged in rows. The pitch between the lenses may be 20-100 μm, preferably 20-70 μm, more preferably 20-50 μm.

The composite optical film of the present invention may be applicable to an apparatus with an LCD such as a television, a notebook computer, a monitor, a hand-held electronic apparatus (e.g. a mobile phone), a camera, a personal digital assistance (PDA) and the like for providing brighter images.

An electronic apparatus (e.g. PAD, mobile phone, television, monitor, notebook computer, refrigerator, etc.) comprises a flat panel display including a composite optical film of the present invention. The flat panel display comprises: a display module showing an image according to image data; and a backlight module projecting light onto the display module. In the embodiment, a composite optical film is disposed between the display module and the backlight module, a diffusion sheet is disposed between the display module and the composite optical film, and a control circuit directs the image data to the display module for generating an image according to the image data.

The foregoing embodiments of the invention have been presented for the purpose of illustration. Although the invention has been described by certain preceding examples, it is not to be construed as being limited by them. They are not intended to be exhaustive, or to limit the scope of the invention. Modifications, improvements and variations within the scope of the invention are possible in light of this disclosure. 

What is claimed is:
 1. An optical film having a structured light input surface and a light output surface opposite to the structured light input surface, comprising: a substrate having a first surface and a second surface opposite to the first surface, wherein the first surface of the substrate defines the light output surface of the optical film, wherein at least one portion of a plurality of particles are disposed in the substrate; and a structured layer having a third surface and a fourth surface opposite to the third surface, wherein the third surface of the structured layer is over the second surface of the substrate and the fourth surface of the structured layer defines the structured light input surface of the optical film.
 2. The optical film according to claim 1, wherein at least one portion of the plurality of particles protrude from the first surface of the substrate such that the first surface of the substrate becomes a rough surface.
 3. The optical film according to claim 1, wherein at least one portion of the plurality of particles are disposed in a first surface region of the first surface, wherein the thickness of the first surface region is less than the thickness of the substrate.
 4. The optical film according to claim 1, wherein at least one portion of the plurality of particles are disposed in a second surface region of the second surface, wherein the thickness of the second surface region is less than the thickness of the substrate.
 5. The optical film according to claim 1, wherein the third surface of the structured layer is on the second surface of the substrate.
 6. The optical film according to claim 1, wherein the structured layer comprises a plurality of microstructures each of which extends substantially along a first direction.
 7. The optical film according to claim 6, wherein each of the plurality of microstructures is a prism structure or a lens structure.
 8. An optical assembly, comprising: an optical film according to claim 1; and a light guide disposed over the structured light input surface of the optical film.
 9. The optical assembly according to claim 8, wherein the light guide has a structured surface facing the structured light input surface of the optical film, wherein the structured layer of the optical film comprises a plurality of first microstructures each of which extends substantially along a first direction and the structured surface of the light guide comprises a plurality of second microstructures each of the extends substantially along a second direction.
 10. The optical assembly according to claim 9, wherein the first direction and the second direction are not parallel.
 11. An optical film having a structured light input surface and a light output surface opposite to the structured light input surface, comprising: a PET substrate having a first surface and a second surface opposite to the first surface, wherein the first surface of the PET substrate defines the light output surface of the optical film, wherein at least one portion of a plurality of particles are disposed in the PET substrate; and a structured layer having a third surface and a fourth surface opposite to the third surface, wherein the third surface of the structured layer is over the second surface of the PET substrate and the fourth surface of the structured layer defines the structured light input surface of the optical film.
 12. The optical film according to claim 11, wherein at least one portion of the plurality of particles protrude from the first surface of the PET substrate such that the first surface of the PET substrate becomes a rough surface.
 13. The optical film according to claim 11, wherein at least one portion of the plurality of particles are disposed in a first surface region of the first surface, wherein the thickness of the first surface region is less than the thickness of the PET substrate.
 14. The optical film according to claim 11, wherein at least one portion of the plurality of particles are disposed in a second surface region of the second surface, wherein the thickness of the second surface region is less than the thickness of the PET substrate.
 15. The optical film according to claim 11, wherein the third surface of the structured layer is on the second surface of the PET substrate.
 16. The optical film according to claim 11, wherein the structured layer comprises a plurality of microstructures each of which extends substantially along a first direction.
 17. The optical film according to claim 16, wherein each of the plurality of microstructures is a prism structure or a lens structure.
 18. An optical assembly, comprising: an optical film according to claim 11; and a light guide disposed over the structured light input surface of the optical film.
 19. The optical assembly according to claim 18, wherein the light guide has a structured surface facing the structured light input surface of the optical film, wherein the structured layer of the optical film comprises a plurality of first microstructures each of which extends substantially along a first direction and the structured surface of the light guide comprises a plurality of second microstructures each of the extends substantially along a second direction.
 20. The optical assembly according to claim 19, wherein the first direction and the second direction are not parallel. 